This invention relates to a core of a light-water reactor particularly provided with a large shut-down margin for the reactor core.
The core of a light-water reactor generally comprises a number of fuel assemblies each constructed by a plurality of fuel rods which are arranged in a regular fashion, and a light water as a coolant also acting as a moderator flows between the respective fuel rods from a lower portion towards an upper portion thereof to remove the heat generated by the fuel rods. Accordingly, the light water is highly heated under high pressure when the light-water reactor is operated or run with a high output power or a rated output power.
With the reactor core of the type described above, most of the control rods are withdrawn from the reactor core during the operation of the nuclear reactor, whereas all the control rods are inserted into the core during the reactor shut-down time. Even when a control rod having the largest reactivity is withdrawn from the reactor core due to a certain reason, the shut-down condition has to be safely maintained with an appropriate shut-down margin.
The concentration (enrichment) of a fissile nuclide contained in a fuel used in the reactor should be increased from an economical view point. The nuclear fission is easily caused by increasing the enrichment of the fuel, which will result in a smaller shut-down margin of the core. The lowering of the core shut-down margin may cause a condition where the reactor is not shut-down in a situation requiring a reactor shut-down. A sufficient shut-down margin should be thus ensured for the reactor, but the ensuring of the sufficient shut-down margin may go against the economical requirement. Taking these facts into consideration, in the conventional technique, a burnable poison is added in the fuel or boron solution is added in the coolant in order to ensure the sufficient shut-down margin, and on the other hand, an improvement for responding to the increasing economical requirement has also been required.
Concerning a boiling-water reactor (BWR), steam voids are formed in the area of the location of the fuel assemblies except the lowest portions thereof and the voids move up towards the upper portion of the reactor core, and accordingly, the void fraction in the BWR becomes high towards the upper portion of the reactor core. As a result, the moderation characteristics for neutrons is lowered and hence the fission rate is also lowered. In other words, the burning progresses at the lower portion of the reactor core and the burning is delayed at the upper portion thereof. In order to obviate this phenomenon; that is, in order to suppress the lowering of the output power at the upper portion of the reactor core, the enrichment of the fissile nuclide contained in the fuel disposed at the upper portion of the reactor core is increased.
However, the increasing of the void fraction at the upper portion of the reactor core and the increasing of the enrichment of the fissile nuclide at the upper portion of the reactor core will result in reduced subcriticality at the upper portion of the reactor core during a core shut-down time of the reactor. On the other hand, in order to increase the burnup and elongate the operation cycle of the reactor for improving the economical requirement, it will be desired to further increase the enrichment of the fuel. However, these facts result in the further reduction of the subcriticality at the upper portion of the reactor core, and finally, there may exist a situation where the reactor is not shut down. Because of this problem, in the conventional technique, it is considerably difficult to increase the burnup in the reactor core.